Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1999-04-27
2003-09-09
Phu, Phoung (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S364000, C375S369000, C370S512000
Reexamination Certificate
active
06618452
ABSTRACT:
BACKGROUND
The present invention generally relates to radio communications systems, and more particularly to frame and frequency synchronization of bursts received via a dispersive channel.
Performing frame and frequency synchronization of transmissions that have been received over unknown frequency-selective channels (i.e., dispersive channels that cause Inter-Symbol Interference (ISI)) is a problem that calls for different solutions than are used when the transmission takes place over a non-selective channel. For time synchronization on frequency-flat (i.e., non-selective) channels, time synchronization via peak detection in the receiver is usually performed by a correlation filter that detects a specific correlation sequence that is inserted by the transmitter. But this procedure will not produce a distinct peak for transmission over unknown frequency-selective (ISI) channels. In other words, the formerly good (optimized) correlation properties are destroyed by the convolution of the originally transmitted signal (including the specific correlation sequence) with the unknown channel impulse response. Apart from this problem, this preamble is not useful for performing carrier frequency synchronization.
For continuous transmission, such as in broadcast applications, the receiver can average the synchronization parameters like frame start position and frequency offset over several preambles in order to obtain a very accurate and reliable frame and carrier frequency synchronization result.
A severe problem in other systems, such as in wireless Asynchronous Transfer Mode (ATM) scenarios, arises as a result of the packet-oriented transmission and the mostly non-continuous traffic. This requires mostly burst synchronization schemes that allow a reliable single-shot frame- and carrier frequency synchronization to be performed.
For frame and carrier frequency synchronization of spontaneous transmissions taking place over unknown frequency-selective fading channels, a special preamble structure has been proposed which consists of some channel symbol sequence that is repeated one or more times, so that periodicity is introduced into the transmitted signal. This is described, for example, in Pierre R. Chevillat, Dietrich Maiwald, and Gottfried Ungerboeck, “Rapid Training of a Voiceband Data-Modem Receiver Employing an Equalizer with Fractional-T Spaced Coefficients”,
IEEE Transactions on Communications
, vol. 35, no. 9, pp. 869-876, 1987 (henceforth “[CMU87]”); Stefan A. Fechtel and Heinrich Meyr, “Fast Frame Synchronization, Frequency Offset Estimation and Channel Acquisition for Spontaneous Transmission over Unknown Frequency-Selective Radio Channels”,
Proceedings of the International Symposium on Personal, Indoor and Mobile Radio Communications
(
PIMRC
'93), PP. 229-233, Yokohama, Japan, 1993 (henceforth “[FM93]”); Stefan A. Fechtel and Heinrich Meyr, “Improved Frame Synchronization for Spontaneous Packet Transmission over Frequency-Selective Radio Channels”,
Proceedings of the International Symposium on Personal, Indoor and Mobile Radio Communications
(
PIMRC
'94), pages 353-357, The Hague, Netherlands, 1994 (henceforth “[FM94]”); and Uwe Lambrette, Michael Speth, and Heinrich Meyr, “OFDM Burst Frequency Synchronization by Single Carrier Training Data”,
IEEE Communications Letters
, vol. 1, no. 2, pp. 46-48, 1997 (henceforth “[LSM97]”). This type of preamble is referred to herein as a “repetition preamble”. Examples of these conventional repetition preamble structures are given in
FIGS. 1
a
and
1
b
. In the conventional repetition preamble depicted in
FIG. 1
a
, the transmitted signal is replicated in the regions designated A and B, and the guard region, G, is usually a copy of the rightmost part of the A region. The conventional repetition preamble depicted in
FIG. 1
b
is similar, but here the transmitted signal is replicated more than once; that is, the signal is identical in each of the regions A, B and C, with the guard region, G, again usually being a copy of the rightmost part of the A region. In each case, the replicated regions (i.e., A and B, or A, B and C) are contiguous to one another. The data to be transmitted in the frame is arranged so that it follows all of the replicated preamble regions.
After convolution of the periodic signal part with the (finite) impulse response of the unknown frequency-selective (ISI) channel, the received signal in the regions A and B (or A, B and C for the case of
FIG. 1
b
) will again exhibit some similarity, assuming that the preamble part G is chosen to be sufficiently long. This is true, even though the shape of the received signal in these regions can be completely different from the transmitted one due to the frequency selectivity (time dispersivity) of the channel. The only difference between the received signal in regions A and B (and B and C) will be a phase shift that is proportional to the carrier frequency offset.
Thus, the receiver can detect the correct starting position of the received signal by processing the received samples, including performing a signal correlation, given that the preamble samples are spaced apart by the discrete periodicity interval k
0
. This is described in Jan-Jaap van de Beek, Magnus Sandell, Mikael Isaksson and Per Ola Börjesson, “Low-Complex Frame Synchronization in OFDM Systems”,
Proceedings of the International Conference on Universal Personal Communication
(
ICUPC
'95), pp. 982-986, Tokyo, Japan, 1995 (henceforth “[vdBSIB95]”); Magnus Sandell, Jan-Jaap van de Beek, and Per Ola Börjesson, “Timing and Frequency Synchronization in OFDM Systems Using the Cyclic Prefix”,
Proceedings of the International Symposium on Synchronization
, pp. 16-19, Essen, Germany, 1995 (henceforth “[SvdBB95]”); Timothy M. Schmidl and Donald C. Cox, “Low-Overhead, Low-Complexity [Burst] Synchronization for OFDM”,
Proceedings of the international Conference on Communications
(
ICC
'96), pp. 1301-1306, Dallas, Tex., USA, 1996 (henceforth “[SC96]”); and [LSM97].
To utilize the conventional repetition preambles of
FIGS. 1
a
and
1
b
, the signal parts in regions A and B (and B and C and eventually A and C) are processed to obtain the desired synchronization parameters time and carrier frequency. The exploitable periodicity interval(s) is (are) illustrated by the lines
101
,
103
and
105
. A minimum (or maximum) value of the timing metric occurs not only at the correct time position, but also in a wide range around it. Hence, these conventional preambles suffer from the same ambiguity problem as the correlation-sequence technique in ISI channels. (In this context, ambiguity should be understood as a blurred extreme point of the timing metric.) If additionally a high noise power is present at the receiver input, the probability of an error in time synchronization is high, resulting in a very high variance of the timing estimate. It should be noted that, as explained in [SvdBB95] and [SC96], the argument of the correlation result between A and B at the correct timing instant offers an estimate for the frequency offset. Thus, it can be exploited for frequency synchronization purposes. With respect to this point, the repetition preamble offers at least one advantage over the correlation sequence preamble.
SUMMARY
It is therefore an object of the present invention to provide apparatuses and methods for performing frequency synchronization of received signals.
It is a further object of the present invention to provide apparatuses and methods for performing frame synchronization of received signals.
The foregoing and other objects are achieved in methods and apparatuses for transmitting and receiving a sequence of data samples. In accordance with one aspect of the invention, a sequence of data samples is transmitted by initially transmitting a first preamble comprising a sequence of preamble samples, then transmitting the sequence of data samples, and then subsequently transmitting
Huber Johannes
Müller-Weinfurtner Stefan
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